1. Field of the Invention
The present invention relates to a small and high performance group of lenses for an optical system that is suitable for use mainly for small photographic devices such as digital cameras.
2. Description of the Related Art
In recent years, in optical equipment such as digital still cameras and video projectors, small and high performance zoom lenses are used.
In a combination lens such as a zoom lens that is used in such optical equipment, compactness and high resolution are required.
Due to this, the applicant of this patent application has filed a patent application for an invention for specifying combination conditions for a small and high performance zoom lens for digital still cameras which is an inexpensive combination lens with a small number of lenses which makes up a zoom lens (Japanese Unexamined Patent Publication No. 2003-057542).
In addition, in these days, light-transmitting ceramics have been developed. These light-transmitting ceramics have higher refractive indices than those of optical glass, and their hardness and flexural strength exhibit higher values than those of optical glass. This has allowed to produce lenses which are suitable for the tendency of making optical systems small in size and thin in thickness.
As has been described above, since light-transmitting ceramics have the higher refractive indices and greater hardness and flexural strength, when used for lenses, the light-transmitting ceramics become optimal for use in making a optical system in optical equipment smaller in size.
In the light-transmitting ceramics, however, since there exists a difference in light transmission properties in wavelength in visible areas, images look as being tinged with yellow, and since bubbles referred to as voids exist in a crystal grain interface, there is caused a problem that the visibility of images formed by ceramic lenses is reduced compared with that by lenses of optical glass.